This invention relates generally to radiographic inspection of aircraft fuselages and more particularly to methods and systems for inspecting aircraft fuselages without a-priori knowledge of interfering structures.
An aircraft fuselage typically comprises a grid of circumferential frame members and longitudinal stringers covered by a skin of lightweight sheet metal. The skin is ordinarily attached to the frame members and stringers by means of rivets or the like. To ensure passenger comfort at high altitudes, aircraft are provided with cabin pressurization systems that produce near sea-level air pressure breathing environments in the aircraft cabin. The application of cabin pressure causes the skin, frame members and stringers to expand slightly. When the pressure is removed, the skin, frame members and stringers return to their normal shape. Although the pressure differentials involved are relatively small, the repeated cycles of stress imposed on the fuselage structure by the pressurization and depressurization sequence that occurs during each flight can lead to fatigue and crack formation. This fatigue damage is often assisted by corrosion of the fuselage structures.
Fatigue cracks by nature can be extremely small in size and difficult to detect. The cracks are normally so small that routine pressurization of the aircraft cabin will not result in detection because the tiny cracks will not cause a detectable pressure loss in the aircraft. The combined effect of corrosion and cyclic stress can also cause looseness around the rivets and/or rivet cracking. If not detected, this condition could result in skin separation from the frame members and stringers.
Traditionally, aircraft fuselage inspection relies largely on visual inspection techniques. These techniques rely heavily on human ability and are limited by ambient lighting conditions, environmental effects, and the inspector""s physical and mental limitations such as eye vision corrections, time constraints, mental attitude, concentration and judgment. Furthermore, visual inspection techniques require extensive disassembly of the aircraft. This approach is thus time consuming, labor intensive and expensive.
Radiography is another approach to aircraft fuselage inspection that has been proposed. While this approach can reduce the amount of aircraft disassembly required with traditional visual inspections, internal cabin objects can significantly complicate x-ray images, thereby masking defects and making their identification and quantification more difficult. These objects include overhead bins, bulkheads, air masks, oxygen plumbing, lights, electrical wiring, fasteners, lavatory and galley fixtures and so on. If the precise location of such interfering objects is known, viewing angles can usually be determined to allow the areas of interest to be imaged without interference. Some of these interfering objects are in known fixed positions. Other objects vary significantly in location from one aircraft to another. For example, electrical wiring and oxygen plumbing are flexible in nature and do not assume a fixed location. Thus, without sufficient a-priori knowledge of interfering structure location, it is difficult to plan or predict viewing angles that will avoid interference. In which case, the initial inspection will provide images where the field of view has been obstructed. This requires the affected areas to be re-inspected from another angle and perspective, which leads to additional inspection expense and time.
Accordingly, there is a need for a method and apparatus for radiographic inspection of aircraft fuselages that permits all or most of a fuselage to be accurately inspected without a-priori knowledge of interfering structure locations.
The above-mentioned need is met by the present invention, which provides a system and method for radiographic inspection of an aircraft fuselage. The system includes a radiation source located on one side of the fuselage and a plurality of radiation detectors located on another side of the fuselage. The radiation detectors are located in known positions relative to the radiation source so as to receive radiation from the radiation source at different angles. The system further includes manipulators for moving the radiation source and the radiation detectors in a coordinated fashion. The system processes the radiation detected by the radiation detectors so as to display stereoscopic images of areas of interest of the fuselage. The stereoscopic images are obtained by first irradiating the fuselage and the radiation detectors with the radiation source to detect a first set of images of the fuselage from multiple angles, repositioning the radiation source and the radiation detectors with respect to the fuselage, and then irradiating the fuselage and the radiation detectors with the radiation source to detect a second set of images of the fuselage. The multiple sets of images are used to produce the stereoscopic images.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.